FIELD OF THE INVENTION
When electronic circuits are embodied as integrated circuits on a semiconductor chip it is sometimes necessary to connect higher voltages than the supply voltage and also to connect negative voltages on the chip to the circuits or parts thereof. In this context, the high positive or negative voltages can either be fed to the chip in addition to the customary supply voltages or be produced from the supply voltages by means of charge pumps on the chip. At all times it is necessary to ensure reliable and rapid switching, and the voltage drop across the switch is to be kept as low as possible.
Thus, it is necessary in the new kind of flash memories, and also in the future in standard EEPROMs, to connect a negative word line voltage to a word line in order to select it. In this case, non-selected word lines are kept at a potential of 0V.
In accordance with U.S. Pat. No. 5,319,604 (EP 0 456 623 A2), the gate of a p-MOS transfer transistor is negatively poled with a pulsed, high positive voltage by means of a capacitor in a voltage inverting circuit. The result is that the negative voltage passes from a charge pump to the word line of a memory. Because of leakage currents in the connections, this circuit must be operated in the burst mode. This means that repeating pulses are necessary up to the end of the programming operation, resulting in a relatively high power loss.
Since the logic circuits which are usually also present on a memory chip in addition to the memory cells are also realized using MOS technology and such circuits are thus operated on a standard basis with purely positive voltages, insulation of the negative voltage with respect to the substrate of the semiconductor chip must be ensured. This can be achieved, for example, by means of insulated wells, using so-called "triple-well" technology.
In a p-type substrate a p-well for n-channel transistors is formed thereby in a deep n-well and thus insulated from the p-type substrate. Such a deep well is obtained, for example, by means of a high-energy implantation with typically 1 to 3 MeV with phosphorus as the doping material or by means of lower energy and during an extremely long diffusion time.